This invention relates to a sol-gel process for producing thin films of luminescent material, and to the thin films so produced. More particularly, the invention relates to a sol-gel process in which a non-luminescent gelled film may be converted to a polycrystalline luminescent film by heating below the firing temperature used for conventional powder phosphors.
Recent increasing interest in high brightness, small cathode ray tubes (CRTs) for projection type displays has generated new and more stringent demands for the phosphor coatings, including higher resolution (for large screen displays), and higher thermal stability to withstand required high electron beam excitation densities.
Conventional powder phosphor layers show limited resolution due to light scattering and reflection from the particles. In addition, such layers show very poor thermal conductivity, due in most part to their porosities caused by the relatively small contact areas between particles. Thus, such layers are unable to easily dissipate the heat generated by high electron beam excitation densities.
Single crystal layers, on the other hand, have excellent thermal conductivities due to their lack of porosity, but because of the lack of grain boundaries or other light scattering sources, can have up to 80 percent of their luminescence trapped due to multiple internal reflections between the layer surfaces.
An attractive compromise between a particulate powder layer and a single crystal layer might be offered by a relatively dense polycrystalline layer. Not only would it have excellent thermal conductivity, but it would have little or no light trapping through internal reflections.
However, the production of dense, polycrystalline luminescent layers by known techniques such as sputtering, plasma spraying and chemical vapor deposition, is difficult to achieve in practice, requiring complex equipment and careful control of process parameters. In addition, the choice of materials suitable for use with these various techniques is fairly limited. See, for example, Studer et al., J. Opt. Soc. Am., vol. 45, no. 7, July, 1955, p. 493; and U.S. Pat. No. 3,961,182.
Several investigators have formed luminescent layers by spraying a solution of the constituents onto a heated substrate to form a film, and then heating the film to promote or enhance crystallization. For example, cathodoluminescent films have been formed by this technique by Gilliland et al., Electrochemical Technology, vol. 5, no. 5/6, 1967, p. 303.
The density and adhesion of such a layer has recently been improved by closely controlling the size of the particles brought into contact with the substrate. See European Patent Specification 0 003 148. However, the brightness of such a layer is not as high as would be desired for the most demanding applications, such as projection TV.
Various techniques have been disclosed for producing mixed oxide ceramics and glasses using sols and or gels. For example, in British patent 1,266,494, mixed oxide microspheres are formed by dispersing an aqueous metal salt solution in a non-miscible organic liquid, and adding a base to induce gelling of the dispersed aqueous phase by hydrolysis. The gelled particles are then separated from the organic phase and heated to effect solid state reaction.
In U.S. Pat. No. 4,278,632, silica-titania binary glasses are formed on a substrate by: first partially hydrolyzing one metal alkoxide by adding water to a dilute organic solution of the alkoxide; then reacting this solution with another metal alkoxide solution to polymerize the reactants; then coating the solution on a substrate; drying the coating; and heating the coating at a temperature of about 400 to 1000 degrees C. to convert the coating to a glass. Similar techniques for glass formation are disclosed in U.S. Pat. Nos. 4,472,510; 4,420,517 and 4,419,115. However, none of these patents suggests that their processes can be used to make luminescent materials.
It is an object of the present invention to readily produce dense, polycrystalline luminescent thin films from a variety of luminescent materials.
It is another object of the invention to produce such luminescent thin films without the need for complex equipment.